1. Field of the Invention
The present invention relates to an apparatus and a process for forming a deposited film. In particular, the present invention relates to an apparatus and a process for forming a deposited film which are capable of forming SiO.sub.2 or SiN film with high quality and uniformity on a substrate.
2. Related Background Art
Deposited film formation apparatuses play an important role in manufacture of semiconductor devices and electronic circuits, particularly of ultra LSIs. For example, plasma CVD apparatuses are employed to form as SiN films for use of a final protective film; plasma CVD apparatuses or atmospheric pressure CVD apparatuses are employed to form SiO.sub.2 film for interlayer insulation; and sputtering apparatuses are employed to form thin aluminum films for wirings.
Regarding SiO.sub.2 films for interlayer insulation, as the devices are being made finer, three-layer structure has come to be used in which spin-on-glass (SOG), which is excellent in step-covering property, is interposed between SiO.sub.2 films for insulation which are formed by plasma CVD or atmospheric pressure CVD. However, this type of interlayer insulation film is liable to cause cracks therein owing to shrinkage after post-heat-treatment of SOG. Therefore the film has to be formed in several steps in order to suppress the occurrence of cracks. As a result, there is a problem of increasing step number. For one-step formation of an SiO.sub.2 film having excellent step-covering property, a method of atmospheric pressure CVD is being investigated in which O.sub.3 and tetraethyl orthosilicate (Si(OC.sub.2 H.sub.5).sub.4, abbreviated to TEO, also called tetraethoxysilane) are used as starting gaseous materials. In the atmospheric pressure CVD, however, the reaction proceeds mainly by surface reaction, so that the reaction is incomplete as a whole. This method may sometimes cause cracks in the film or corrosion of aluminum wiring on the film by contamination with a large amount of radicals of hydroxyl, ethyl, etc., especially at a low temperature below 400.degree. C. required to form interlayer insulation films. In contrast, the plasma CVD enables formation of SiO.sub.2 films with better quality than the atmospheric pressure CVD at temperature below 400.degree. C.
In the atmospheric pressure CVD and plasma CVD, irradiation of light during film formation is known to accelerate the film formation reaction. Since, in principle, the deposited film-forming apparatuses which utilize light enable treatment at a lower temperature with less damage, they are expected to be used and they are now starting to be applied to cleaning and annealing processes.
In the formation of a deposited film by the method utilizing light, a window for introducing the light is provided generally in a reaction vessel. However, the film is formed also on the face of the window, thereby blurring the window and reducing greatly the illuminance of the incident light introduced into the reaction vessel. A method for preventing the blurring of the window such as a grease-coating method, a gas-purging method and the like are reported. FIG. 9 and FIG. 10 are schematical views of structures of the conventional deposited film-forming apparatuses employing respectively a grease-coating method and a gas-purging method to prevent the blurring of the light introduction window.
In the deposited film-forming apparatus as shown in FIG. 9, a transparent light introducing window 61 constitutes the upper face of a cylindrical reaction vessel 50, and an illumination system 60 as the light source is provided in contact with the upper face of the light introducing window 61. An evacuation opening 59 connected to an evacuation pump not shown in the drawing is provided at the bottom of the reaction vessel 50. Above the evacuation opening 59 of the reaction vessel 50, a supporting member 53 is provided for supporting a substrate 52 to be treated. The starting gases for the reaction are fed into the reaction vessel 50 through a ring-shaped starting gas-introducing tube 58. In this apparatus, the starting gas-introducing tube 58 is placed by the side of the substrate 52 to feed the starting gases from the periphery of the substrate 52, so that the light from the illumination system 60 is not intercepted and is illuminated uniformly onto the substrate 52. Onto the inside face of the light introducing window 61 of the reaction chamber, a transparent grease is applied. With this constitution of the deposited film-forming apparatus, film formation and blurring on the light introducing window is prevented since the inorganic thin films conventionally used for semiconductor devices are less liable to adhere onto grease.
The deposited film-forming apparatus as shown in FIG. 10 differs from the one in FIG. 9 in that a perforated separation plate 62 is provided to separate the reaction chamber into an upper space and a lower space of the chamber vessel 50: the upper space being a purge chamber 54 and the lower space being a reaction chamber 51. The substrate 52, the supporting member 53, the starting gas-introducing tube 58, an evacuation opening 59 are placed in the reaction chamber 51. Purge gas introducing tubes 57 are placed in the purge chamber 54. The purge gas does not participate directly in the reaction, but serves to keep the pressure in the purge chamber 54 higher than that of the reaction chamber 51. In the deposited film-forming apparatus shown in FIG. 10, the light-introducing window 61 is not coated with grease. In this deposited film-forming apparatus, diffusion of the starting gases into the gas purge chamber 54 is prevented by maintaining the pressure in the purge chamber 54 higher than that in the reaction chamber 51, thereby the film is formed on the substrate 52 without formation of a film on the light introducing window 61.
In the deposited film-forming apparatuses employing light irradiation as described above, the starting gas is introduced from the periphery of the substrate so that light is irradiated on the substrate uniformly. Therefore the apparatuses have a disadvantage that the pressure and composition of the starting gas are not uniform on the surface of the substrate and the film thickness may vary depending on the pressure of film formation. Further, in the case where grease is applied to prevent the film formation on the light introducing window, the apparatuses have the disadvantage that a grease component evaporated by light irradiation is liable to contaminate the formed film, and if the illuminance of the light is increased to promote the film formation, the grease itself may cause the blurring of the window. In the gas purge method, when the pressure is insufficient to purge the starting gases, a film may deposit on the separation plate to cause blurring and thereby form a non-uniform deposited film even if the separation plate is transparent.